1. Field of the Invention
The preferred embodiments are directed to making nano-identification sample property measurements, and more particularly, using Peak Force Tapping® mode AFM and IR electromagnetic excitation for localized photothermal nanoID imaging and spectroscopy.
2. Description of Related Art
The interaction between a sample under test and radiated energy can be monitored to yield information concerning the sample. In spectroscopy, dispersion of light from a sample into its component energies can be measured and, for example, intensity plotted as a function of wavelength. By performing this dissection and analysis of the dispersed light, users can determine the physical properties of the sample, such as temperature, mass, and composition.
Notably, making spectroscopic measurements with a spatial resolution on the nanoscale is continuing to improve. However, despite ongoing progress in the development of imaging techniques with spatial resolution beyond the diffraction limit, simultaneous spectroscopic implementations delivering chemical specificity and sensitivity on the molecular level have remained challenging. Far-field localization techniques can achieve spatial resolution down to about 20 nm by point-spread function reconstruction but typically rely on fluorescence from discrete molecular or quantum dot emitters, with limited chemically specific information.
One technology for improving spectroscopic measurement performance is scanning probe microscopy. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and causing the tip to interact with the surface of a sample with appropriate forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15. Scanner 24 generates relative motion between the probe 14 and sample 22 while the probe-sample interaction is measured. In this way images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY scanner that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e. the force resulting from tip/sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.” In a recent improvement on the ubiquitous TappingMode™, called Peak Force Tapping®(PFT) Mode, feedback is based on force as measured in each oscillation cycle.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid, or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Infrared (IR) spectroscopy is a useful tool in many analytical fields such as polymer science and biology. Conventional IR spectroscopy and microscopy, however, have resolution on the scale of many microns, limited by optical diffraction. It has become apparent that it would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. That way, information about the composition of the sample, such as location of different materials or molecular structures, would be possible.
Conventional infrared (IR) spectroscopy is a widely used technique to measure the characteristics of material. In many cases, the unique signatures of IR spectra can be used to identify unknown material. IR spectroscopy is performed on bulk samples which gives compositional information but not structural information. Infrared spectroscopy allows collection of IR spectra with resolution on the scale of many microns. Near-field scanning optical microscopy (NSOM) has been applied to some degree in infrared spectroscopy and imaging. While there have been some advancement with NSOM, the field is still in need of a sensitive and reliable commercial instrument. At this time, no widely available instrument provides routine IR spectra with resolution below the diffraction limit. One technique based on use of an AFM to produce such localized spectra is described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” (PTIR) Optics Letters, Vo. 30, No. 18, Sep. 5, 2005. The technique is also discussed in US Pub. No. 2009/0249521 (The '521 publication), the entirety of which is expressly incorporated by reference herein. Those skilled in the art will comprehend the details of the technique in the publication but the technique will be described briefly herein for clarity.
Referring to the '521 publication, in PTIR, infrared radiation is incident on a region of a sample. At a wavelength absorbed by the sample, the absorption will typically cause a local increase in temperature and a rapid thermal expansion of the sample. A probe is arranged to interact with the sample and transducer to generate a signal related to the IR energy in the region under the probe tip. “Interact” means positioning the probe tip close enough to the sample such that a probe response can be detected in response to absorption of IR radiation. For example, the interaction can be contact mode, tapping mode or non-contact mode. An associated detector can be used to read one or more probe responses to the absorbed radiation. The induced probe response may be a probe deflection, a resonant oscillation of the probe, and/or a thermal response of the probe (e.g., temperature change). For probe deflection and/or resonant oscillation of the probe, appropriate detectors can include split segment photodiodes along with any associated amplification and signal conditioning electronics. In the case of a thermal response, the appropriate detector may comprise, for example, a Wheatstone bridge, a current and/or voltage amplifier and/or other associated electronics to sense, amplify, and condition the thermal signal from the probe. The probe response is then measured as a function of the wavelength of incident radiation to create an absorption spectrum. From the spectra, material in the sample can be characterized and/or identified.
As noted in the '521 publication, an AFM set-up used for the published work on IR spectroscopy is shown. The sample is mounted on a ZnSe prism, or prism made from other suitable materials, which does not absorb the radiation of interest. A pulsed IR source, in this case a Free Electron Laser beam, is directed into the prism. The prism is made at an angle such that the beam is in Total Internal Reflection in order for the beam to be propagative in the sample and evanescent in the air. Thus, only the sample is significantly exposed to the laser radiation, and the AFM probe is minimally exposed to the beam. The Free Electron Laser (FEL) is an IR source that is both variable in wavelength and has a pulsed output. Free Electron Lasers are large expensive facilities. The probe is placed at a point on the sample by the scanner and is held at an average height by feedback electronics. Both the vertical and lateral deflection signal, as well as the feedback signal, may be monitored.
When the FEL is pulsed, the sample may absorb some of the energy, resulting in a fast thermal expansion of the sample as shown in FIG. 3. This has the effect of a quick shock to the cantilever arm, which, if the ability of the cantilever to respond to this shock is slower than the shock, will result in exciting a resonant oscillation in the cantilever arm. Because the absorbed energy is ideally contained within the sample, this shock is due primarily to rapid sample expansion, as minimal IR energy is absorbed by the cantilever itself. Although the probe is kept in contact with the surface by the feedback electronics, the resonant signal is too fast for the feedback electronics, but can be observed directly from the photodetector. Thus the cantilever rings down while still in contact with the surface, an effect called “contact resonance”. The absolute deflection, amplitude, and frequency characteristics of the contact resonance vary with the amount of absorption, as well as other properties, such as the local hardness, of the localized area around the probe tip, for example, by analyzing the ringdown and/or the Fourier transform (FFT) of the ringdown events. Also, depending on the direction of the expansion, vertical resonances, lateral resonances, or both can be excited. By repeating the above process at varying wavelengths of the FEL, an absorption spectra on a localized scale is achieved. By scanning the probe to various points on the sample surface and repeating the spectra measurement, a map of IR spectral surface characteristics can be made. Alternatively, the wavelength of the FEL can be fixed at a wavelength that is characteristic of absorption of one of the components of the sample. The probe can then be scanned across the sample surface and a map of the location of that component can be generated.
Although the set-up as described is promising, there is no real possibility of commercializing the same. First, the IR light source used, the Free Electron Laser, is a very large and expensive facility, as noted. Moreover, alternative benchtop sources of IR radiation have been limited by one or more characteristics that have made them unsuitable for a widely available instrument.
As further noted in the '521 publication, the apparatus described in the literature suffers from other limitations beyond the expensive and stationary IR source. The apparatus employs a bottoms-up illumination scheme that requires a sample to be placed on a specially fabricated IR transmitting prism. In addition to being costly and easy to damage, this arrangement requires special sample preparation techniques to prepare a sample thin enough such that the IR light can penetrate the sample to reach the probe. Further, the actual signals generated can be small, thus requiring averaging of the signal and limiting the bandwidth of the technique. More sensitivity is required to address a wider range of potential samples.
The applicant associated with the '521 publication contends that the system disclosed therein can be used to obtain IR spectra from highly localized regions of a sample, allowing discrimination and/or identification of the composition of a micro or nano-sized region of a sample. The system may be used for mapping the variations in IR absorption over a wider area of a sample, by imaging the energy absorbed at one or more wavelengths. One additional problem, however, is leakage of heat from a region that actually is absorbing the excitation energy, to a region that is not. When using PTIR, as the sample is heated (i.e., a region of interest exhibits absorbing characteristics), the heat may, and often does, leak toward surrounding regions of the sample. If the probe scans a location that is indirectly heated, the instrument may identify that location as being responsive to the IR excitation and conclude it is absorbing, when in fact it was not. Clearly, this can lead to compromised data and/or poor resolution. Independent of its ability to provide localized spectroscopy, an improved IR microscopy instrument that can provide efficient localized spectroscopic measurements was desired.
Despite ongoing progress in the development of imaging techniques with spatial resolution beyond the diffraction limit, spectroscopic implementations delivering chemical specificity and sensitivity on the molecular level have remained challenging. Again, far-field localization techniques can achieve spatial resolution down to about 20 nm by point-spread function reconstruction but typically rely on fluorescence from discrete molecular or quantum dot emitters, with limited chemically specific information. Scanning near-field optical microscopy (SNOM) provides sub-diffraction-limited resolution through the use of tapered fibers or hollow waveguide tips. However, aperture-limited and wavelength-dependent fiber throughput reduces sensitivity, generally making SNOM unsuitable for spectroscopic techniques that have low intrinsic signal levels.
In scattering-type SNOM (s-SNOM) external illumination of a sharp (metallic or semi-conducting) probe tip can enhance sensitivity, spectral range, and spatial resolution. Chemical specificity can be obtained through the implementation of, for example, IR vibrational s-SNOM, tip-enhanced coherent anti-Stokes Raman spectroscopy (CARS), or tip-enhanced Raman scattering (TERS). Here the antenna or plasmon resonances of the (noble) metal tips can provide the necessary field enhancement for even single-molecule sensitivity.
In the standard implementation, however, the direct illumination of the tip apex results in a three-to-four orders of magnitude loss in excitation efficiency, related to the mode mismatch between the diffraction-limited far-field excitation focus and the desired tens of nanometers near-field localization, as determined by the tip apex radius. The resulting loss of sensitivity, together with a far-field background signal, often limit contrast and may cause imaging artifacts, presenting challenges for the general implementation of a wider range of spectroscopic techniques in s-SNOM.
Given the interest in spectroscopy-related characteristics of samples on a much smaller scale, an improved instrument was desired to expand the range and efficiency of performing optical imaging and spectroscopy for chemical identification on the nanoscale.